Monday, October 08, 2007

That Elusive Reservoir ...

...of cattle Tb, is the subject of John Daykin and Dr. Thomas's excellent overview of the ISG final report. In the third part of extracts from their Opinion piece in the Veterinary Times, they explore the evidence for that, as presented by chairman of the ISG (Independent Scientific Group) Professor John Bourne:

Professor Morrison and Professor Bourne presented data and opinions concerning testing procedures and a perceived persistence of infection in cattle that they could not substantiate. Professor Bourne is clearly of the opinion that in the West Country herds remain persistently infected, with undetected infection in cattle. He could not offer any proof for this opinion, and had to tacitly accept that this phenomenon could equally result from constant herd re-infection from a common wildlife source (badgers), a totally different proposition.

This is the firmly held opinion of the bTb vets within the SVS,(now re-branded 'Animal Health') and we repeat that there is no evidence for infection being persistently maintained within cattle due to the poor sensitivity of the skin test. Every other country has successfully eradicated bTb using it. Only the presence of a wildlife reservoir, providing constant, pernicious reinfection makes it appear inadequate. But that of course, provides employment opportunities for many, as we have pointed out before. Who would turn down a predicted growth of 20 per cent annually, and a bottomless cash pit of opportunity?

Mr. Daykin and Dr. Thomas point out in their Opinion essay, that both Professors Bourne and Morrison have stated unequivocally, "that the skin test leaves a “substantial” number of undetected infected animals, but that they could not offer any proof for this assumption". In fact the opposite scenario has been proved time after time in past 'trials' and clearances. With no action on cattle whatsoever except 60 day skin tests and removal of reactors, but combined with a cull of infected badgers as indicated by experienced Wildlife operatives - using coloured beads to track the target culprits - bTb just melted away. Often for several years.

How is it that this same test which nearly eradicated bTB by 1985 is suddenly now being blamed for its poor sensitivity to explain the failure to control the current epidemic? The cynical would suggest that such an opinion accords with the ISG’s view that constantly refining cattle controls will bring this epidemic under control, and would also explain their enthusiasm for the gamma IFN test. This view would thus be self-serving.

They point out that such measures as gamma IFN "will not reduce the impact of bovine TB" and stress that the "single common source of the majority of breakdowns, the wildlife reservoir in badgers (and to a much lesser extent, deer), must be tackled if we are to stand any chance of controlling this disease". And conclude "If we delay now, the battle will be lost."

Both authors point out that although numbers of cattle proving reactors to the intradermal skin test is hugely increasing, that does not mean that the source is within the cattle herd[s], or that the test is in any way flawed:

Whilst it is indisputable that many more cattle are now infected with bovine TB than 20 years ago, testing at 60 day intervals in infected herds removes reactors so fast that there is little likelihood that these individuals are significantly infectious and a danger to other cattle. This has been demonstrated in experimental transmission studies (8). Very few ‘open’ cases of TB are found at post-mortem, and there is a virtual absence of safe scientific evidence to demonstrate infectivity in the field from ‘closed’ cases of disease. Certainly cattle translocate the disease out of endemic areas, but rigid application of the skin test has resolved these situations in the absence of a wildlife reservoir. There is absolutely no evidence to prove that cattle to cattle transmission maintains infection within herds in endemic areas, as indicated above.

The authors then point out the reason for bTb testing of cattle - as if those of us on the receiving end were in any doubt. This serious, debilitating, highly infectious disease is not confined to cattle or badgers. It is zoonotic. And they point out that it is "indisputable that infection is now spilling over into other wildlife (9), farmed and domestic species, and that eventually man may again be at risk from this grade 3 pathogen if a holistic approach to control is not rapidly adopted. The recent report by the Health Protection Agency of six linked cases of M.bovis in immuno-compromised men in Birmingham is a reminder that this risk is real and not imagined."

Spillover was always going to be the lever which finally tips Defra's intransigence over this disease into some type of action. Forget cattle. They are expendable - as Trevor Lawson, media person to the Badger Trust so quaintly pointed out on a Radio 4 programme. But try explaining to Mrs. Rural-Edge-of-Suburbia why her cat has succombed to bTb, or to the mothers of children rolling in infected badger excrement on school playing fields, why they are at risk. Already spillover - not from cattle - is noted in alpacas, free range pigs, companion-type cattle and domestic pets. It will only get worse.

Shooting the messengers - the sentinel tested cattle - is a short term fix for what is a far more serious problem lurking in GB's countryside, but one which is increasingly entering its gardens and leisure areas.